The present invention relates to combustors for gas turbines and particularly relates to a flow controller for promoting both velocity and temperature uniformity of combustion products flowing to the inlet of a catalyst.
Reduced emissions of nitrogen (NOx) and hydrocarbon compounds in gas turbines is an ever-present goal. There are a number of different methods of reducing these emissions, all of which have certain drawbacks in terms of reduced turbine efficiency and increased costs. For example, steam can be injected into the combustor to reduce combustor flame temperature and hence minimize or eliminate the reaction of nitrogen in the air at elevated temperatures which produces the emissions. Steam injection, of course, requires ancillary costly equipment. Another method of reducing unwanted emissions is to provide a catalyst in the combustion products flow stream before exhausting to atmosphere. The catalytic reaction of the combustion products and the catalyst produce a number of harmless components and hence reduce unwanted emissions. A catalyst could also be used to enable combustion of very lean mixtures (usually below the flammability limit). The catalyst partially converts the fuel in a flame-less reaction such that the local temperatures within the catalyst and in downstream homogeneous combustion remain below the minimum temperature for NOx formation.
When using catalytic combustion to reduce emissions, it is highly desirable that the fuel/air distribution should be uniform at the inlet to the catalyst. Absent this flow uniformity in both velocity and temperature, uneven combustion with consequent reduction in combustor efficiency and increased emissions may occur. It will be appreciated that the output from the preburner section of a combustor has a center peaked flow distribution. That is, the flow distribution has a parabolic profile with the peak generally along the axial region of the combustor. Thus, the peak flow is characterized by both high velocity and high temperature. Additionally, the openings in the combustor liner tend to squeeze the flow toward the center axis of the combustor. Previous attempts to provide a uniform distribution of flow have included the use of perforated plates and honeycomb-type flow conditioners at the preburner exit. Also, multiple tubular-type venturi devices have been proposed in efforts to achieve a uniform flow. However, even utilizing multiple venturis such as described and illustrated in U.S. Pat. No. 4,845,952 does not entirely cure the problem of providing a uniform flow of fuel/air mixture to the catalyst inlet because the air flow can vary from venturi to venturi, with different mass flows, for example, peaking, along the central axial region of the combustor. Accordingly, there is a need for a device to promote flow uniformity in one or the other, and preferably both, of velocity and temperature flow parameters at the inlet to the catalyst.